Precipitated silicas, silica gels with and free of deposited carbon from caustic biomass ash solutions and processes

ABSTRACT

Disclosed are amorphous precipitated silicas, silica gels, and amorphous carbons derived from biomass and methods of producing them with and without adhered or deposited amorphous carbons produced by acidifying a caustic silicate solution produced by caustic digestion of biomass ash containing silica with and without activated carbon, the ash being obtained from thermal pyrolysis of the biomass, the acidifying effective to produce a slurry of the precipitated silica and silica gels with and without adhered or deposited amorphous carbon, and separated from the slurry the precipitated silicas and silica gels with and without the adhered or deposited amorphous carbons. The properties of the precipitated silica with adhered or deposited carbon being within the range as utilized in rubber compositions thereby avoiding the blending of silica and carbon components for such use. The precipitated silicas and silica gels without adhered or deposited carbon having metal contaminants present in low concentrations which when used in formulation of chemical-mechanical-planerization slurries used in polishing silicon wafers in the manufacture of computer chips and other electronic devices do not contaminate the wafer and final chip product and the other electronic devices.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. applicationSer. No. 09/159,809, filed Sep. 23, 1998, which is acontinuation-in-part application of U.S. application Ser. No.08/977,524, filed Nov. 24, 1997, now U.S. Pat. No. 5,858,911, which is adivisional application of U.S. application Ser. No. 08/677,875, filedJul. 10, 1996, now U.S. Pat. No. 5,714,000, which is acontinuation-in-part application of U.S. application Ser. No.08/642,925, filed May 6, 1996, abandoned.

FIELD OF THE INVENTION

[0002] The present invention relates to precipitated silicas and silicagels, having adhered or deposited activated carbon and free of carbon,from a caustic biomass ash solution and their production.

BACKGROUND OF THE INVENTION

[0003] Commercially available precipitated silicas are produced throughan acidulation process utilizing a caustic silicate solution, such assodium silicate solution, with a mineral acid, such as sulfuric acid.Commercially available caustic silicate solutions are conventionallymade by fusing high purity soda ash and silica sand in furnaces attemperatures of 1300° to 1500° C. and higher to produce a solid glass.The silicate solution is made by dissolving the glass with steam and hotwater. This is the foundation of all commercial processes for makingsodium or other soluble silicate solutions today. Both processes arevery energy intensive, thus very expensive, and the silicates generallycontain metal contaminants found in the earth in amounts from about 500to 10,000 ppm. Processes for producing precipitated silicas aredescribed in detail in U.S. Pat. Nos. 2,657,149; 2,940,830; 4,157,920;4,495,167; and 4,681,750, the entire disclosures of which areincorporated herein by reference, including the processes for producingprecipitated silicas and the properties of the product. In general, acidand silicate solutions are added to a reactor and by manipulation of theprocess conditions, the chemical and structural properties can becontrolled. After completion of the precipitation reaction, the solidprecipitate is filtered, washed to remove soluble byproducts, dried andmilled to the desired size.

[0004] Silica gels, another form of amorphous silica with slightlydifferent properties, are produced in a similar manner as previouslydescribed, however, in lower pH solutions. The process for commerciallyproduced silica gels, entails treating a solution of soluble metalsilicate, usually sodium silicate, with a strong mineral acid such assulfuric or hydrochloric acid. Since the gel phase does not settle out,silica gel is customarily described as a non-precipitated, homogenousmixture of colloidal amorphous silica particles. The end product is thenwashed to remove soluble salts, dried, and reduced to a suitableparticle size range. There are generally two types of silica gels,namely, hydrogels and aerogels. Hydrogels are prepared as previouslydescribed and aerogels are usually prepared from unrefined hydrogels bydisplacing the water with an alcohol, which is recovered during thedrying process. Silica gel, a glassy material, has immense internal porearea, giving it the capacity to absorb large quantities of moisture aswell as other substances.

[0005] Precipitated silicas with added carbon adhered or deposited onthem are utilized for various rubber applications, which require highstrength and abrasion resistance, such as tires and industrial products.The current methodology for using combinations of silica and carbon asreinforcing agents in rubber entails blending the solid components intothe rubber composition, which usually requires the addition ofdispersants and coupling agents to achieve a homogenous mixture. Inpractice, the carbon is normally selected from carbon blacks that arecommercially available and conventionally used in tires, treads, hoses,etc. Examples include carbon blacks with ASTM designated N-numbers,which are well known to those skilled in the rubber compounding art.These carbon blacks are produced commercially by subjecting heavyresidual oil feedstock to extremely high temperatures in a carefullycontrolled combustion process. This production process is very energyand labor intense, which results in high manufacturing costs.

[0006] In practice, the commonly used siliceous compounds (silicas)employed in rubber compounding applications are typically precipitatedsilicas, such as those obtained by the acidification of solublesilicates, i.e., sodium silicate. The preferred silicas include thosemarketed by AKZO, PPG, DuPont, Rhone-Poulenc, Huber and Degussa. Also,coupling agents capable of reacting with both the silica surface and therubber elastomer are utilized to cause the particulate precipitatedsilica to have a reinforcing effect on the rubber.

[0007] As mentioned previously, precipitated silicas and silica gels areutilized as reinforcing fillers in many applications, particularly, inthe rubber industry. For various rubber applications, which require highstrength and abrasion resistance, such as tires and industrial products,a combination of silica and amorphous carbonaceous components areutilized. Carbon black and silica with or without a coupling agent arecommonly used as reinforcing fillers for various rubber products,including the treads, undertreads, and sidetreads of tires; industrialhoses, conveyor belts, rolls; rubber shock absorbers; and the like. Theuse of silica and carbon as reinforcing fillers for elastomers,including sulfur curable rubber, is well known to those skilled in suchart.

[0008] U.S. Pat. No. 5,610,216 discloses a rubber composition with thecombination of silica and carbon black utilized as reinforcing filler,with a ratio of silica to carbon black in the range of 3/1 (75% silicaand 25% carbon) to about 30/1 (96.77% silica and 3.23% carbon). Therubber composition comprises about 25 to about 100 parts of reinforcingfiller composed of silica and carbon black per 100 parts by weight ofrubber (phr).

[0009] As previously mentioned, carbon black is produced commercially bysubjecting heavy residual oil feedstock to extremely high temperaturesin a carefully controlled combustion process. By adjusting conditions inthe combustion process, dozens of commercial grades with varyingstructure and particle size, are produced. Carbon black structuralproperties such as, surface area and pore volume, are evaluated andmeasured using methods similar to those utilized for precipitatedsilicas and silica gels. The principle measurement of a carbon black'sstructure, i.e., the degree of interlinkage between particles, isusually determined by the DBP (dibutyl phthalate) oil absorption inaccordance with ASTM D2414, with values in milliliters absorbed per 100grams of carbon (ml/100 g). The measurement of surface area iscustomarily performed by a BET (Brunauer, Emmett, Teller) nitrogenadsorption test method, ASTM D3037 or ASTM D4820 with values in squaremeters per gram of carbon (m²/g). Some manufacturers use ASTM D3765,CTAB (cetyltrimethylammonium bromide) adsorption for surface area, whichresults in values in m²/g identical to the BET values, in most cases.Also, some manufacturers utilize ASTM D1510, Standard Test Method forCarbon Black-Iodine Adsorption Number, as a measurement of surface area.For example, a higher Iodine Number, expressed in mg/g, is indicative ofsmaller particle size and higher surface area, which typically indicatesa better reinforcing carbon black for rubber elastomers.

[0010] Iodine Numbers and DBP Numbers together with ASTM designatedN-numbers for carbon blacks, may be found in The Vanderbilt RubberHandbook, 13th Edition (1990). The DBP number is indicative of structurewith a higher number indicating a higher structure and usually largeraggregate size. The BET nitrogen adsorption number is indicative ofsurface area with a higher number indicating a higher surface area and,usually, a smaller particle size.

[0011] U.S. Pat. Nos. 5,168,106; 5,679,728; and 5,798,405 disclosecarbon blacks suitable for the aforementioned uses, with structureproperties as follows: DBP (dibutylphthalate) Adsorption Numbers rangingfrom 80 to 135 ml/100 g, BET Nitrogen Adsorption Numbers ranging from 20to 300 mg/g, and Iodine Numbers ranging from 25 to 300 mg/g.

[0012] U.S. Pat. No. 5,809,494 discloses a silica gel composition with acarbonaceous component attached to a gel component. The carbonaceouscomponent may be selected from the group consisting of: carbon blacks,carbon fibers, activated carbons and graphite carbons. If necessary, thecarbonaceous component nay be modified so that it will attach to the gelcomponent, thereby, increasing its water dispersibility. Suitable gelcomponents for use in the gel compositions include metal oxide gels suchas silica gels, titania gels, alumina gels and the like. The amount ofcarbonaceous component included in the gel composition will depend onthe intended end use. Generally, amounts of 1 to 99%, by weight of thecarbonaceous component, may be utilized in the gel composition. If gelcompositions with lower bulk density are desirable, then amounts of 1 to50%, by weight, of the carbonaceous component are utilized. If higherbulk density gels are desirable, 50 to 99%, by weight, of thecarbonaceous component are utilized. The gel compositions includingcarbonaceous components may be utilized in applications known to thoseof ordinary skill in the art, which include the following: Insulationapplications, including thermal, electrical, and acoustical insulation;particulate additive applications, including thickeners in pigments,inks, and food products; flatting agents in paints and coatings; fillersin cements, adhesives, and rubber compositions; reinforcing agents inpolymers and natural or synthetic rubber compositions; adsorbents forliquid, gas or vapor adsorption processes; catalyst supports forpowdered metal or metal oxide catalytic materials; membranes forselective liquid, gas or vapor separations; filters for filtration ofparticulates; radiation detectors; heat resistant coatings as in thermalbarrier coatings; and low dielectric materials.

[0013] Gel precursors, suitable for use in the gel composition of theU.S. Pat. No. 5,807,494, include metal oxide precursors known in theart, such as: SiO₂ in alkoxide, sodium silicate and colloidal forms;TiO₂ in alkoxide and colloidal forms; Al₂O₃ in alkoxides, colloidal,sodium aluminate and salts forms. The choice of a particular precursoris made based on the type of gel desired. As will be recognized by thoseof ordinary skill in the art, whether a particular gel composition isdesirable for use in a particular application will depend on thecharacteristics of the gel composition, such as amount of carbonaceousmaterial incorporated and the bulk density of the composition.

[0014] Examples 23-28 of U.S. Pat. No. 5,807,494 are directed to silicagels produced from a sodium silicate precursor and less than or equal to50%, by weight (solids), of a carbonaceous component. Three carbonblacks were used in these examples with the properties listed in Table1, page 10 as: Nitrogen surface areas (ASTM D3037) from 24 to 560 m²/g,DBP oil absorption (ASTM D2414) from 70 to 132 ml/100 g and averageprimary particle size (ASTM D3849) from 16 to 130 nanometers. Prior toinitiating gel formation, a specific amount of a particular carbon blackwas added to a sol (a liquid colloidal suspension or solution)consisting of commercially available sodium silicate (SiO₂/Na₂O ratio of3.22:1) and 2M sulfuric acid, with a pH of about 3. Gelation wasinitiated by controlled addition of 1M sodium hydroxide until the pH ofthe sol increased to about 5. After washing free of salts, solventexchanging, and drying, representative samples were evaluated by arub-off technique and scanning electron microscope (SEM) photographs, todetermine whether the carbonaceous material is attached to the gelcomponent.

[0015] U.S. Pat. No. 5,679,728 discloses a carbon black having silicaadhered to or deposited on the surface thereof (referred to as “silicasurface-treated carbon black”) utilized in a rubber composition for tiretreads, undertreads, and side treads, which gives a low fuel consumptionand superior durability, without reducing the braking performance andother tire performance. Also, disclosed is to provide a silicasurface-treated carbon black which suppresses a rise in the electricalresistance which is a defect of silica, and improves the dispersibility.The silica-treated carbon black preferably has a nitrogen specificsurface area (N₂SA) of 20 to 300 m²/g and a DBP oil adsorption of 90 to180 ml/100 g. The silica surface-treated carbon black was prepared bythe following method. A carbon black (DBP adsorption of 115 to 119ml/100 g) slurry, prepared by an ordinary method, was warmed to 90° C.,then diluted JIS No. 3 sodium silicate was added over 4 hours by aconstant delivery pump, the pH was maintained at 5 to 10 by dilutesulfuric acid and an aqueous solution of sodium hydroxide, wherebysilica was deposited on the surface of the carbon black. Next, the pHwas adjusted to 6 and the solution was allowed to stand for 6 hours,then was filtered, rinsed, and dried to obtain the desired substance.The content of the silica was changed by adjusting the amount of sodiumsilicate added.

[0016] The content of the silica in the silica surface-treated carbonblack was found analytically and the properties, including nitrogenspecific surface area and the iodine adsorption were determined. Forsilica surface-treated carbon samples prepared in the above manner, thesilica content ranged from 2 to 74 wt %; the nitrogen specific surfacearea ranged from 94 to 193 m²/g and the iodine adsorption ranged from 2to 137 mg/g. Various rubber compositions for tire tread, undertread, captread, and side tread uses were prepared using the silicasurface-treated carbon black, then tested and the results compared tothose obtained from rubber compositions compounded with normal carbonblack and silica reinforcements. The results clearly showed the rubbercompositions with silica surface-treated carbon black provided anexcellent grip and low rolling resistance. Further, the electricalresistance was reduced and the dispersion of the silica surface-treatedcarbon black in the rubber composition was improved.

[0017] U.S. Pat. No. 5,916,934 discloses an elastomeric compoundincluding an elastomer and a silica coated carbon black, and optionallyincluding a coupling agent. This patent teaches a carbon black coatedwith silica, is expected to provide advantages over carbon, silica, ormixtures thereof in an elastomer. While any carbon black may be used,the desirable properties are determined by analytical methods know inthe art. These properties include: particle size and specific surfacearea; aggregate size, shape, and distribution; and chemical and physicalproperties of the surface. Furthermore, examples of useful silicasinclude: silica, precipitated silica, amorphous silica, vitreous silica,fumed silica, fused silica, silicates such as aluminosilicates, andother Si-containing fillers such as clay, talc, wollastonite, and thelike. Silicas are commercially available from such sources as CabotCorporation, PPG Industries, Rhone-Poulenc, and Degussa AG.

[0018] From the prior art, it is apparent that precipitated silicas,silica gels, and carbons used, singularly or in some combination, needto possess certain properties relating to structural characteristics,surface area, porosity, adsorption and absorption, surface activity,wetting characteristics, dispersibility in aqueous solutions, and bulkdensity levels. For example, silicas combined with carbons asreinforcing fillers in rubber products, elastomers, and other polymers,generally have higher surface areas and oil absorption values. Anotherimportant feature for these uses is dispersibility in the aqueous rubbercompositions. Several of the aforementioned patents teach that thedispersibility of the silica-carbon components is enhanced by usingsilica surface-treated carbon black, carbonaceous components attached tothe silica gel component, silica coated carbon black, and silanecoupling agents. While the silica in the rubber reinforcing componentdoes not necessarily have to be physically or chemically bonded to thesurface of the carbon black, it is advantageous for the silica to beadhered to or deposited on the carbon black surface.

[0019] A recently developed application for precipitated silica incolloidal form is in formulation of Chemical-Mechanical-Planarization(CMP) slurries used in polishing silicon wafers in the manufacture ofcomputer chips and other electronic devices. U.S. Pat. Nos. 3,922,393,4,260,396, 4,588,421, 5,100,581, 5,230,833, 5,527,423, 5,603,805,5,860,848, and 5,891,205, as examples, teach the use of colloidal silicasolutions with various concentrations, modifications and additives,which are particularly useful as chemical mechanical polishing slurriesin the process of polishing and planarization of silicon wafers for thesemiconductor industry.

[0020] As known in the art, CMP polishing slurries generally consist ofa chemical agent which is corrosive to the material to be removed with asolution pH to cause controlled surface dissolution and some type ofabrasive particles to mechanically remove material from the surface. Itis also known that the electrical performance of the finishedsemiconductor chips can easily be affected by contaminates acquired bythe wafers during processing. The use of silica slurries that arecontaminated with trace transition metals, alkali and alkaline earthmetals, aluminum, and the like have caused problems when used in waferpolishing. Contaminating metals of particular concern include alkalimetals such as sodium (Na) and potassium (K), alkaline earth metals suchas calcium (Ca) and magnesium (Mg), and transition metals such as iron(Fe), copper (Cu), manganese (Mn), nickel (Ni), zinc (Zn) and the like.Another metal which can cause difficulties is aluminum, particularly ifFe, Ni, Cu, Mn, or Cr are also present. If the metals are present inhigh enough concentrations, the CMP silica slurries often cannot meetthe requirements for the final product.

[0021] It is generally agreed that CMP materials and solutions whichcome in contact with the wafer surface should have the lowest possiblemetals content to prevent possible contamination of the wafer and finalchip product.

[0022] Precipitated silica particles in colloidal form for use asabrasives in CMP slurries are typically produced by acidification ofcommercial sodium silicate solutions, which are readily available.Commercial silicates are produced from the earth mined raw materials,sand and soda ash, by fusion at high temperatures in open hearthfurnaces. This produces molten silicate glass, which is then cooled,fractured, and charged into vessels where it is dissolved under pressureby hot water and steam to form aqueous silicate solutions. Since thesand and soda ash raw materials contain various impurities found in theearth and the furnace insulation surfaces also transfer metal oxides,commercial silicates for industrial use, such as set forth above,generally contain metal contaminates in amounts from about 500 to 10,000ppm which cause metal contamination as set forth below.

[0023] As known in the art, to minimize the possibility of metalcontamination in or on the silicon wafers resulting from polishingoperations, the manufacturers of CMP slurries have developed varioustreatment processes for metals removal from the silicate solutions andsilica based CMP slurries. These expensive and time-consuming treatmentprocesses, for example, include: treatment of the silica powderparticles with acid to remove the metals while applying ultrasonicvibrations; hydrolyzing the silica compound containing the metals whilein contact with a finely divided carbonaceous carrier on which the oxideis deposited and then separating the oxide from the carbon; removal ofthe counterions using ion exchange and then backadding ammoniumhydroxide and ammonium carbonate to form stable products; diluting acolloidal silica slurry with water, then exposing it to a cationexchange resin in acid form to remove all sodium values, then addingoxalic acid to form an oxalate-containing low pH silicic acid, thenexposing the silicic acid solution to an anion exchange resin inhydroxide form to replace all negatively charged species with hydroxideions, then exposing the hydroxide-neutralized silicic acid solution to acation exchange resin in acid form to replace all positively chargedspecies with hydrated protons thereby forming a low metals silicic acidsolution, then chilling, adding ammonium hydroxide for stabilization,heating, and reacting to form a dilute, low metals silica solution.

[0024] It is known that several CMP slurry manufacturers have attemptedto develop colloidal silica slurries with lower metal impurities byobtaining high purity sand and soda ash raw materials for the productionof microelectronic grades of sodium silicate. These attempts haveresulted in lowering total metals contaminates to about the 250 ppmlevel, however, the costs have made it commercially unattractive.Another technique employed by some manufacturers includes the use ofchelating agents in the slurry mixture to inhibit migration of metals tothe wafer surface.

[0025] Commercially available rice hull ash currently is produced bygasification or by combustion or by incineration of rice hulls in afurnace. Gasification is the conversion of the hydrocarbon orcarbohydrate components in a solid fuel into gases through theapplication of heat. Combustion is the act or process of burning or achemical change, especially oxidation, accompanied by the production ofheat and light. Incineration is the act of consuming by burning toashes. Thermal pyrolysis is a chemical change that occurs in a substancethrough the application of heat. For convenience, the term “thermalpyrolysis” includes gasification, combustion, incineration, and any andall forms of heat which produces rice hull ash and amorphous carbon fromrice hulls. Any process in which thermal pyrolysis is used to producerice hull ash and amorphous carbon from rice hulls may be used in thepresent invention.

[0026] It has been recognized that certain agricultural byproducts orwaste materials have varying quantities of biogenic silica, that is,silica which is developed, assimilated or occurs in the cell structuresof living organisms such as plants. These byproducts, commonly referredto as “biomass”, are principally rice hulls, rice straw, wheat straw,and sugarcane baggase. Other plants that contain biogenic silica,include equisetum (“horsetail weeds”), certain palm leaves (“palmyrapalm”), and certain bamboo stems. The biogenic silica in theseagricultural byproducts and plants lacks distinct crystalline structure,which means it is amorphous with some degree of porosity.

[0027] Dry rice hulls are comprised of about 60% cellulose andhemicellulose, 20% lignin, 19% silica, 0.5% nitrogen and sulfur, and0.5% mineral elements such as phosphorus, magnesium, manganese, iron,potassium, sodium, aluminum, titanium, and calcium. When rice hulls arecombusted, the solid material or ash remaining comprises about 20% ofthe starting quantity of hulls by weight and consists primarily ofsilica, minerals and any uncombusted carbon.

[0028] Dry sugarcane bagasse is typically comprised of about 6% sugar(carbohydrates) and 94% fibrous material (cellulose, hemicellulose,lignin, silica, and minerals). Chemical analyses of bagasse ash yields:60-73% SiO₂ , 3-6% Al₂O₃ , 2-3% CaO, 5-6% Fe₂O₃, 3-4% K₂O, 3-4% MgO,3-4% Na₂O, 4-5% P₂O₅, and 4-17% uncombusted carbon.

[0029] Agricultural waste materials or biomass have potential usefulfuel value and are used as low grade fuel to produce steam andelectricity in a number of locations, especially near rice milling andsugarcane processing operations. Direct combustion and incineration havebeen utilized for many years as an expeditious method to dispose of ricehull waste. In the usual incineration of rice hulls, furnaces have beendesigned to operate at extremely high temperatures without regard to theform of silica produced by this incineration. The phase diagram ofsilicon dioxide indicates that a transition from the amorphous,non-crystalline form to the crystalline forms known as tridymite andcrystobalite takes place at temperatures above 2000° F. (1093° C.) whenthe silica is in pure state. However, the incineration of biogenicmaterial, such as rice hulls at temperatures in the 1800° F. to 2000° F.range for any prolonged exposure period, has lead to the formation ofcrystalline silica because the transition temperature from amorphous tocrystalline is reduced by the presence of other components of theoriginal rice hulls.

[0030] U.S. Pat. Nos. 3,889,608 and 3,959,007 disclose a furnace andprocess for the incineration of biogenic material, such as rice hulls toproduce useable energy and a highly reactive amorphous form of silica inthe ash. In the current incineration or direct combustion process, rawrice hulls are exposed to elevated temperature in an excess of air inthe combustion zone of a cylindrical furnace, and the ash iscontinuously removed from the bottom. The hulls are incinerated at a gasmass temperature of between 1250° F. (677° C.) and 1500° F. (815° C.) atrelatively high levels of turbulence under conditions whereby thetemperature of the rice hulls does not exceed about 1300° F. (704° C.).Gas mass temperatures of between 1250° F. (677° C.) and 1350° F. (732°C.) are preferred when a crystalline free ash is desired. Upon leavingthe furnace, the ash is rapidly cooled to provide ease in handling. Theincineration or combustion of rice hulls and other biogenic materialsare time-temperature related, and burning of them under these conditionsproduces biogenic ash, such as rice hull ash having carbon particlesfrom the burning of the hulls which activates the carbon. Incinerationof the hulls in this manner produces from about 3 percent to about 14percent by weight of activated carbon. Also, when rice hulls and otherbiogenic materials are incinerated in this manner, the silica in the ashremains in a relatively pure amorphous state rather than in thecrystalline forms known as quartz, tridymite, or crystobalite.

[0031] The significance of having the silica in an amorphous state isthat the silica maintains a porous skeletal structure which providesbetter chemical reactivity and solubility during operations such ascaustic digestion of the ash. From a safety standpoint, a commonly knownhealth hazard which has been associated historically with the inhalationof crystalline silica dusts is silicosis. In 1997, a working group ofthe International Agency for Research on Cancer (IARC) published amonograph classifying inhaled crystalline silica from occupationalsources as carcinogenic to humans, and categorized it as an IARC Group 1agent. The Occupational Safety and Health Administration (OSHA)regulations and its OSHA Hazard Communication Standard, stateright-to-know laws, and other applicable federal, state, and local lawsand regulations on crystalline silica establish Permissible ExposureLimits (PELs) for airborne crystalline silica. OSHA has publishedgeneral industry PELs for three different forms of crystalline silica.Cristobalite and tridymite are forms of crystalline silica, lessabundant than quartz, that have lower PELs than quartz. These PELs forcrystalline silica in general industry are listed in the “Code ofFederal Regulations,” 29 CFR 1910.1000, “Air Contaminants,” under TableZ-3, “Mineral Dusts”.

[0032] In the incineration process, all of the oxidation or combustiontakes place rapidly and, typically in a single chamber where the biomassmaterials are placed in intimate contact with oxygen. This can result incompeting reactions which can produce NO_(x) (oxides of nitrogen),SO_(x) (oxides of sulfur), and other compounds which are potentialenvironmental contaminants. These and several other limitations havebrought about development of gasification type combustion of biomassfuels. Biomass gasification involves the high temperature, about 1450°F. (788° C.), conversion of agricultural wastes, such as rice hulls andsugarcane bagasse, into combustible gases, such as hydrogen, carbonmonoxide, methane, ethane and non-combustibles, such as carbon dioxide,water, and ash. The gases are then burned in a combustion chamber or inthe radiant section of a boiler for production of steam and electricity.The ash is automatically and continuously discharged and cooled in itsdry state. Components of the system normally include a two or threestage gasifier, boiler, steam turbine, generator, condenser and controlsystem.

[0033] U.S. Pat. Nos. 4,517,905 and 4,589,355 disclose a gasifier wherethe carbon content of the ash residue from combustion of agriculturalwastes, such as rice hulls, and the fly ash content of the gaseousexhaust are controlled. The combustion process is performed in atraditional manner using underfire and overfire air to support efficientgasification of the rice hulls or other feedstock, which results inproduction of a combustible gas mixture that is carried through a firetrain to a boiler, steam turbine or other energy recovery system. Avariable feed system, which when manipulated in conjunction with othervariables, can produce ash with carbon contents from about 10% to thehigh 30% range. Operating instructions provide for control of thecombustion chamber temperatures in the range of 1280° F. (693° C.) to1460° F. (793° C.). The corresponding combustion gas (boiler feed)temperatures are in the range of 1360° F. (738° C.) to 1600° F. (871°C.). This combustion chamber temperature range is sufficient todevolatize rice hulls and allow partial combustion of some of the fixedcarbon in the hulls. The specially designed feed system and temperaturecontrol mechanism permit the production of a dry, amorphous ash fromrice hulls.

[0034] Any process in which thermal pyrolysis, including theaforementioned incineration, combustion, and gasification processes, isused to produce biogenic ash, such as rice hull ash and activated carbonfrom them may be used in the present invention. The biogenic silica isobtained by the controlled combustion of biogenic materials so thatsubstantially all of the silica is in an amorphous rather than acrystalline state although minor amounts of crystalline silica can bepresent. While amorphous silica in the ash is preferred, somecrystalline silica can be accommodated by manipulating caustic digestionvariables such as temperature and pressure in the reaction. Generally,in the commercial burning of rice hulls as an energy source, theresulting ash includes about 0.5% to 1.0% of trace metals, such asmagnesium, potassium, iron, aluminum, calcium, titanium, and manganese.The concentration of these metals is dependent upon the soil conditionsand composition in which the rice plants and other biogenic materialsare grown.

[0035] U.S. Pat. No. 5,833,940 discloses the production of liquidsilicates from biogenic silica, by dissolving in a closed containerbiogenic silica, preferably rice hull ash, in a strong alkali solution,preferably sodium hydroxide in the presence of an active carbonmaterial. The production of a caustic silicate solution, such as sodiumsilicate, from biogenic silica in rice hull ash is a caustic digestionprocess. Biogenic material ash, preferably, rice hull ash, is withdispersed activated carbon is heated with a caustic solution, such assodium hydroxide, which reacts with the amorphous silica to createsodium silicate solution. As mentioned previously, the carbon content inthe rice hull ash or other biogenic material can approach the high 30%levels depending on the type of thermal pyrolysis used to burn them. Thecarbon is an inert material during the reaction and excess carbon is notharmful to the reaction. The principal caustic digestion chemicalreaction is characterized as follows:

2NaOH+nSiO₂+H₂O->Na₂O:nSiO₂+H₂O

[0036] where “n” represents the silica/alkali weight ratio

[0037] For the current industry standard sodium silicate liquidsolution, the chemical equation becomes:

2NaOH+3.22SiO₂+H₂O->Na₂O:3.22SiO₂+H₂O

[0038] Present commercial grades of liquid sodium silicates not derivedfrom rice hull ash range in silica/alkali weight ratios from about 1.6to about 3.8. Such ratios are satisfactory for the rice hull ash andother biogenic ash derived liquid sodium silicate in the presentinvention.

[0039] As described in U.S. Pat. Nos. 5,714,000 and 5,858,911, activatedcarbon is generated in quantities ranging from about 3 percent to asmuch as 40 percent by weight in rice hull ash depending on the type ofthermal pyrolysis utilized to burn rice hulls. During the causticdigestion of rice hull ash to produce sodium silicate solution, thebiogenic silica reacts with the alkaline element (sodium oxide in thecaustic solution) and becomes a soluble compound in the silicatesolution. The activated carbon remains an inert material and becomessuspended solids in the silicate solution. Advantageously, it has beendetermined in the present invention, the compositions of the dilute,unfiltered sodium silicate liquids derived from caustic digestion ofrice hull ash with carbon quantities in 3% to 40% by weight range, areideally suited for the production of precipitated silicas with adheredor deposited amorphous carbons. In the event the carbon content of thebiogenic ash or other biomass ash is too high for the intended end use,all of the activated carbon can be filtered out and the desired amountadded to the caustic silicate solution prior to initiating theprecipitation reaction. Specifically, the silica/alkali weight ratios(SiO₂/Na₂O), dissolved silicate solids (Na₂O:nSiO₂), suspended carbonparticles and water quantities in the aqueous solutions are within theranges necessary to provide commercial grade precipitated silicas withadhered or deposited amorphous carbons.

[0040] It would be highly desirable and advantageous to provide novelamorphous precipitated silicas and silica gels with amorphous carbonadhered or deposited on the silica in natural state rather than havingto add carbon black to the amorphous precipitated silica.

[0041] It would also be highly desirable and advantageous to providenovel amorphous precipitated silica with the amorphous carbon filteredout and which has a low metal's content which prevents possible metalscontamination by the CMP polishing slurry of the wafer and final chipproduct, thus providing an electronic grade amorphous silica.

[0042] It would be also highly desirable to provide amorphous carbonsfrom a caustic silica solution produced by caustic digestion of biomassash, preferably rice hull ash.

SUMMARY OF THE INVENTION

[0043] The present invention is directed to amorphous precipitatedsilicas, silica gels and amorphous carbons derived from biomass ash,preferably rice hull ash, and to a process for producing such silicacompounds and carbons. More particularly, in one embodiment of theinvention, the invention is directed to precipitated silicas and silicagels with adhered or deposited amorphous carbons in natural state, whichutilize aqueous caustic silicate solutions of amorphous silicacontaining diffused carbon derived from caustic digestion of biogenicsilica ash, preferably rice hull ash, from the thermal pyrolysis of ricehulls.

[0044] The process of this embodiment of the invention is the productionof the precipitated silicas and silica gels with adhered or depositedamorphous carbons by reacting acidification agents, such as strongmineral acids, with caustic silicate solutions of amorphous silica, suchas liquid sodium silicate, containing diffused carbon derived fromcaustic digestion of biomass ash, such as rice hull ash, from thethermal pyrolysis of rice hulls.

[0045] In the embodiment of the invention in which precipitated silicaswithout adhered or deposited carbons, the carbon is not dissolved in therice hull ash and other biogenic ash silicate solutions; therefore, itcan be removed by conventional liquid/solids filtration or separationequipment as described in U.S. Pat. No. 5,714,000. Then, the activatedcarbon can be treated as disclosed in U.S. application Ser. No.09/159,809 and pure silicas, without adhered or deposited carbon, can beproduced from the clear caustic silicate solutions by the acidulationprocess to provide electronic grade silicas of lower metals content thanthose produced from commercial silicates.

[0046] As mentioned previously, the silica components are precipitatedby acidifying the dilute aqueous caustic silicate solutions with orwithout diffused carbon, generally, by employing a strong mineral acidsuch as sulfuric acid or hydrochloric acid. Other strong mineral acidswhich may be used include; phosphoric acid, nitric acid, and aceticacid.

[0047] The manner in which the foregoing objects and other objects areachieved in accordance with the present invention will be betterunderstood in view of the following detailed description and examples,which form a part of the specification.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] One embodiment of the present invention are amorphousprecipitated silicas or silica gels with adhered or deposited amorphouscarbons separated from a slurry containing them, the slurry produced byacidifying a caustic silicate solution produced by caustic digestion ofbiomass ash containing silica and activated carbon. Advantageously andsurprisingly, no additional carbon is necessary to be added to theprecipitated silicas or silica gels for the industrial applicationspreviously described.

[0049] In another embodiment of the invention, amorphous precipitatedsilicas or silica gels free of carbon separated from a slurry of themproduced by acidifying a caustic silicate solution produced by causticdigestion of biomass ash containing silica and from which carbon hasbeen filtered out, the precipitated silicas and silica gels having metalimpurities that are less than half the concentration levels inprecipitated silicas produced from commercial sodium silicate and,therefore, require significantly less treatment for metal impuritiesremoval to make them suitable for CMP slurries used in polishing siliconwafers used in the manufacture of computer chips and other electronicdevices.

[0050] In general the methods of the invention provide producingamorphous precipitated silicas, silica gels with and without adhered ordeposited amorphous carbons comprising acidifying a caustic silicatesolution produced by caustic digestion of a biomass ash, preferably ricehull ash, containing silica with or free of activated carbon, theacidifying effective to produce a slurry of the precipitated silicas andsilica gels with or without the adhered or deposited amorphous carbons,and separating from the slurry the precipitated silicas and silica gelswith or without the adhered or deposited amorphous carbons.

[0051] More particularly, the methods of the invention comprisesreacting acidification agents with aqueous alkali metal silicates, suchas sodium silicates, derived from caustic digestion of biomass ash,preferably rice hull ash, containing biogenic amorphous silica andactivated carbon. The activated carbon passes through the causticdigestion as an inert material, therefore, it is not dissolved in thesodium silicate solution. When the unfiltered sodium silicate solutions,containing the diffused activated carbon particles, are reacted withacidulation agents, amorphous silica particles are precipitated and theactivated carbon adheres to or deposits on the silica particles. If pureprecipitated silica or silica gel products are desired, the carbonparticles are removed from the biogenic sodium silicate solution byconventional filtration or separation equipment, prior to initiating theacidulation precipitation reaction.

[0052] Accordingly, one embodiment of the present invention utilizes thefollowing process steps:

[0053] 1. A measured quantity of biogenic sodium silicate solutionderived from biomass is introduced into a suitable reactor vesselequipped with agitation, heating, and pH measurement capabilities. Priorto introduction, the properties and composition of the biogenic silicatesolution, including sodium oxide (Na₂O) and silica (SiO₂)concentrations, SiO₂/Na₂O ratio, Na₂O:nSiO₂ soluble solids, carbonsuspended solids, water percent, specific gravity, and pH are determinedby standard analytical methods known in the industry. These propertiesare important to achieve an overall chemical reaction balance regardingthe quantity of acidification agent to utilize.

[0054] 2. The solution is heated to a temperature range of 50° to 55° C.(122° to 131° F.) with agitation, while acidification is initiated bygradually adding the aqueous acid solution. Although any mineral acidmay be used, either sulfuric acid or hydrochloric acid is preferredbased on economic cost factors.

[0055] 3. Acid addition is continued until about 70% of the initialsodium oxide (Na₂O) in the silicate solution is reacted, at which time athick slurry is formed and gelling starts to occur. The acid additiontime period to this point is normally about 22 to 38 minutes, dependingon the silicate composition and acid strength. Also, the reaction masspH can be used as a gelling indicator, since it will decrease from theinitial starting range of 11.0 to 11.5 to the 8.0 to 9.0 range.

[0056] 4. At this time, the acid addition is stopped and the reactionmass is aged for a period of 15 to 30 minutes, while maintainingagitation and a temperature of about 50° C. (122° F.). Acid addition isthen continued at the previous rate until the pH is reduced to about the3.4 to 4.2 range.

[0057] 5. Acidification in this manner results in a pH of about 6.4 to7.9, for a 5% solution of the final dried product in distilled water.While this pH range is suitable for most uses, adjustments can be madefor more alkaline products (pH>8) by simply adding additional sodiumsilicate solution after acidification is completed.

[0058] 6. The slurry is then processed through suitable solids/liquidseparation equipment, such as a vacuum filter unit, centrifuge, orfilter press for recovery of the wet solids or filter cake.

[0059] 7. The wet solids or filter cake contain soluble salts; such assodium sulfates, sodium chlorides, or sodium phosphates produced by theacid reaction with the alkali metal oxide component of the silicate,which are removed by washing with hot water at a temperature of about60° C. (140° F.).

[0060] 8. After washing, the wet solids or filter cake can be dried byany conventional drying methods and equipment, such as convection orradiant heaters, rotary drum dryers, spray dryers, etc.

[0061] 9. The dry amorphous precipitated silica with adhered ordeposited activated carbon particle sizes natural distribution from thedryer are about 62.5% smaller than 180 microns (−80 mesh) and 37.5%larger than 180 microns (+80 mesh). Milling, grinding, or pulverizingcan be performed by any conventional size reduction equipment to producesmaller particle sizes as desired.

[0062] The amorphous precipitated silica with adhered or depositedcarbon produced by the above described embodiment will commonly haveproperties as follows: The silica to carbon ratio (weight basis) will bein the range of about 1.20 /1 to 14.7/1, the pH of a 5% solution will beabout 6.38 to 7.88, the residual soluble salts after water washing willbe about <10 ppm to 540 ppm, the bulk density will be about 17.48 to28.71 pounds/cubic foot, the BET surface area will be about 155 to 267m² /g, and the DBP oil absorption will be about 129 to 223 ml/100 g.

[0063] Another embodiment of the present invention involves theproduction of pure precipitated silicas and silica gel without adheredor deposited amorphous carbons by acidification of the biogenic silicatesolution derived from biomass after filtration removal of the carbonsuspended solids. This embodiment of the present invention utilizes thefollowing process steps:

[0064] 1. The biogenic sodium silicate solution produced by causticdigestion of rice hull ash is pumped from the digestion reactor to afilter, such as a filter press, where the carbon suspended solids areseparated from the liquid solution, thus yielding a clear, homogenousfiltrate free of unreacted silica and carbon solids.

[0065] 2. Advantageously, the reactor used for caustic digestion, afterappropriate clean-out, may be used for acidification of the filteredbiogenic sodium silicate to produce pure precipitated silica or silicagel.

[0066] 3. The filtered sodium silicate solution is then analyzed andtreated in the same manner as outlined in process Steps 1 through 8 ofthe preceding first embodiment.

[0067] 4. The dry, pure amorphous precipitated silica or silica gel(without adhered carbon) particle sizes natural distribution from thedryer are about 76% smaller than 180 microns (−80 mesh) and 24% largerthan 180 microns (+80 mesh) Milling, grinding, or pulverizing can beperformed by any conventional size reduction equipment to producesmaller particle sizes as desired.

[0068] The pure amorphous precipitated silica or silica gel produced bythis second described embodiment will typically have properties asfollows: the pH of a 5% solution will be about 7.1 to 7.8, the residualsoluble salts after water washing will be about 60 ppm to 540 ppm, thebulk density will be about 21.04 to 29.46 pounds per cubic foot, the BETsurface area will be about 205 to 287 m²/g, and the DBP oil absorptionwill be about 171 to 239 ml/100g.

[0069] As described in U.S. Pat. No. 4,157,920, precipitated silicaproduced by prior art methods from commercial sodium silicate solutions,which are useful as reinforcing fillers in rubber and elastomers,generally hold a high percentage of water, i.e., from about 70 to 85% inits wet filter cake. The percent moisture in the filter cake is known aspercent wet cake moisture and generally abbreviated as “% WCM”. Thesolid content of the filter cake is calculated by subtracting the % WCMfrom one hundred. This percent filter cake solids is generallyabbreviated as “% FCS”. When silicas, such as the prior art productshold a high percentage of water, i.e., from about 70 to 85%, they areknown as high structure silicas. The amount of total structural waterassociated with 100 pounds of solid silica content of the filter cake isdefined as “structure index” and abbreviated as S.I. The S.I. iscalculated by the formula:${S.I.} = {\frac{\% \quad {WCM}}{\% \quad {FCS}} \times 100}$

[0070] The total structural moisture content or S.I is a very importantproperty which is directly related to the functional and end useproperties of silica. Prior art silicas, which are high structuresilicas, have S.I.s in the range of 233 to 567. As stated, these silicasare useful as reinforcing fillers in elastomers and rubber. Theamorphous precipitated silicas, with and without adhered or depositedcarbon, of the present invention have S.I.s in the range of 315 to 382,indicative of high structure silicas suitable for reinforcing agents inrubber products, elastomers, and other polymeric compounds.

[0071] The composition of the biogenic sodium silicate utilized in theprecipitation reaction may vary widely without adverse effects. Whilenot being limited thereto, the present preferred compositions andproperties for liquid sodium silicates derived from rice hull ash havingusefulness in this invention for production of amorphous precipitatedsilicas with adhered or deposited carbons are: SiO₂/Na₂O ratios in therange of 2.5/1 to 3.85/1, SiO₂ concentrations in the range of 15% to28%, Na₂O concentrations in the range of 4.0% to 9.5%, Na₂O:nSiO₂soluble solids in the range 18.0% to 38.5%, water concentrations in therange of 55.0% to 81.0%, suspended carbon solids in the range of 1.5% to17.0%, solution specific gravity in the range of 1.128 to 1.300,solution densities in the range of 9.40 to 10.85 pounds per gallon, andpH in the range of 11.2 to 11.6.

[0072] The present invention is further and more particularly describedin the following examples which are intended as illustrative only ratherthan limiting, since numerous modifications and variations will beapparent to those skilled in the art.

EXAMPLES

[0073] Examples 1-7 are directed to the production of precipitatedamorphous silica with adhered or deposited carbon solids from a dilutesodium silicate solution derived from the caustic digestion of rice hullash containing diffused activated carbon particles formed during thethermal pyrolysis of rice hulls. The results are provided in Table Ifollowing Example 7.

Example 1

[0074] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon solids from a dilute sodiumsilicate solution derived from the caustic digestion of rice hull ash(RHA) containing diffused activated carbon particles formed during thethermal pyrolysis of rice hulls. The RHA sodium silicate solution hadthe following properties: SiO₂/Na₂O ratio at 3.27 to 1, Na:Si totalsoluble solids at 26.14% (6.12%Na₂O:20.02%SiO₂), diffused activatedcarbon at 10.25%, in a 63.61% water solution, specific gravity of 1.255and density at 10.47 pounds per gallon, with a pH of 11.23. Theacidulating agent utilized was concentrated sulfuric acid (H₂SO₄), 96.5%in aqueous solution.

[0075] In this example, a one liter laboratory reactor was used toproduce the precipitated silica/carbon product. The reactor was equippedwith a variable speed agitator, heater with temperature measurement andcontrol, and pH measurement via electrode and meter. In this example andthroughout the specification, parts and percentages are by weight unlessotherwise indicated.

[0076] The batch production steps included a quantity of 100 ml (125.5grams) of RHA sodium silicate solution was added to the reactor. Aquantity of 573.5 ml (573.5 grams) of distilled water was added to thesilicate solution and these components were agitated to achieve ahomogeneous mixture. The resulting mixture comprised: 4.70% [Na₂O:3.27SiO₂]+1.84% carbon (suspended solids)+93.46% water. The pH of themixture was 11.07. The mixture was agitated while heating to atemperature range of 50°-55° C. (122°-131° F.). While maintaining thetemperature in the above range, 22.5 ml (39.55 grams) of concentratedH₂SO₄ acid were added to the reaction mass at a rate of 0.6 ml (1.05grams) per minute over a time period of 38 minutes. Gel formationstarted to occur after addition of about 12 grams of acid at about 13minutes elapsed time. The pH of the reaction mass was 8.30 when gellingwas first observed. The gel became a black slurry with continuedaddition of the acid, agitation and heating to maintain the prescribedtemperature range. At the end of the 38 minutes acid addition period,the black slurry, with a pH of 3.93, was allowed to age for completeprecipitation reaction for a period of 15 minutes. After the agingperiod, 205 ml (257 grams) of the dilute RHA sodium silicate solutionwere added over a 20 minutes period (10 ml or 12.55 grams per minute)while continuing agitation and heating to maintain the 50°-55° C.temperature range. The above operations produced 990.5 grams of blackslurry with a pH of 3.90. The material balance closure was minus 0.5%with 995.57 grams input and 990.5 grams output. The material loss wasdue to evaporation, handling and transfer losses.

[0077] Analyses of the unwashed black slurry on a CompuTracliquid/solids analyzer revealed the slurry consisted of 18.25% drysolids and 81.75% liquid (180.76 grams dry solids and 809.73 gramsliquid). Microscopic examination of the unwashed dry solids indicateddistinct silica with attached carbon particles along with scatteredwhite deposits. These deposits were most likely sodium sulfate producedby the H₂SO₄ and Na₂O:3.27 SiO₂ precipitation reaction. A solution of 5%unwashed dry solids in distilled water had a pH of 8.92 with anelectrical conductivity of 1270 microSiemens/cm (uS/cm) which convertsto 850 ppm total dissolved salts (TDS).

[0078] The black slurry was then filtered and the wet solids were washedwith hot (55°-60° C.) distilled water for removal of the soluble saltssuch as sodium sulfate. A total quantity of hot water equal to 5 timesthe starting wet solids weight was utilized. This resulted in removal ofessentially all the sodium sulfate in the solids.

[0079] The washed wet filter cake removed from the filter consisted of20.73% dry silica/carbon solids and 79.27% water. This wet cake wasdried in a standard convection oven at 110° C. for about 3 hours untilconstant weight was achieved.

[0080] The dry silica/carbon product was comprised of 66.1% amorphousprecipitated silica and 33.9% activated carbon. A solution of 5% solidsin distilled water had a pH of 6.95 and total dissolved salts of <10 ppm(<15 uS/cm conductivity). The bulk density was 0.30 g/ml (18.73 poundsper cubic foot). The product had a BET surface area of 220 m²/g, a porevolume of 0.7707 ml/g, and an average pore diameter of 14.0 nanometers.

Example 2

[0081] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon solids from a sodium silicatesolution containing a higher concentration of silicate soluble solidsand less carbon suspended solids than that utilized in Example 1. TheRHA sodium silicate solution had the following properties: SiO₂/Na₂Oratio of 2.83 to 1, Na:Si total soluble solids at 36.97%(9.65%Na₂O:27.32%SiO₂), diffused activated carbon at 1.86%, in a 61.17%water solution, specific gravity of 1.289 and density at 10.75 poundsper gallon, with a pH of 11.43. The acidulating agent was sulfuric acidas used in Example 1.

[0082] The same laboratory reactor and equipment in Example 1 were againutilized in this example.

[0083] The batch production steps were essentially the same used inExample 1 except as follows. A quantity of 155 ml (200.21 grams) of RHAsodium silicate solution was added to the reactor. A quantity of 500 ml(500.53 grams) of distilled water was added to the silicate and theagitator was employed to obtain a homogeneous mixture. The resultingmixture comprised: 6.16% [Na₂O:2.83 SiO₂]+0.31% carbon (suspendedsolids)+93.53% water. The pH of this solution was 10.80. The mixture wasagitated while heating to a temperature range of 50°-55° C. Sulfuricacid addition was started at a rate of 1.40 grams per minute. Gelformation started after addition of 12.64 grams and a thick gel occurredafter addition of 16.36 grams of acid. The elapsed time was 19 minutes.The pH of the reaction mass was 8.94. Acid addition was stopped at thispoint and the black gel mixture was allowed to age for 15 minutes, whilecontinuing agitation and heating to maintain 50°-55° C. temperature.After aging, acid addition was resumed at the same rate until a total of27.12 grams were added. The total elapsed time for acid addition was 34minutes. A black slurry was formed with pH in the range of 3.48-3.53. Anadditional 45.22 grams of RHA sodium silicate were added for pHadjustment. The above operations produced 763.19 grams of black slurrywith material balance closure at minus 1.3% due to evaporation,handling, and transfer losses.

[0084] The black slurry was then filtered and the wet solids washed withhot distilled water (55°-60° C.) for removal of the soluble sodiumsulfate salts. The washed filter cake consisted of 21.56% drysilica/carbon solids and 78.44% water.

[0085] The dry silica/carbon product was comprised of 93.6% amorphousprecipitated silica and 6.4% activated carbon. A solution of 5% solidsin distilled water had a pH of 6.38 and total dissolved salts of <10 ppm(<15 uS/cm conductivity). The bulk density was 0.37 g/ml (23.10 poundsper cubic foot). The product had a BET surface area of 155 m²/g, a porevolume of 0.5429 ml/g, and an average pore diameter of 9.9 nanometers.

Example 3

[0086] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon solids from a sodium silicatesolution containing a higher concentration of carbon suspended solidsthan those in Examples 1 and 2. The RHA sodium silicate solution had thefollowing properties: SiO₂/Na₂O ratio of 2.76 to 1, Na:Si soluble solidsat 27.60% (7.33%Na₂O:20.27%SiO₂), diffused activated carbon at 16.95%,in a 55.45% water solution, specific gravity of 1.298 and density at10.83 pounds per gallon, with a pH of 11.51. The acidulating agent wassulfuric acid as used in the previous examples.

[0087] The laboratory reactor and equipment used in previous exampleswere utilized in this example.

[0088] The batch production steps were as follows. A quantity of 892 ml(1157.82 grams) of RHA sodium silicate was added to the reactor. Nodistilled water was added to the silicate solution which comprised:27.60% [Na₂O:2.76SiO₂]+16.95% carbon (suspended solids)+55.45% water.The pH was 11.51. The solution was agitated while heating to atemperature range of 50°-55° C. Sulfuric acid was added at a rate of 2.5grams per minute while agitating. Gel formation started after additionof 55.50 grams and a thick gel occurred after addition of 69.77 grams ofacid. The elapsed time was 28 minutes. The pH of the reaction mass was8.46. Acid addition was stopped and the black gel mixture was allowed toage for 15 minutes, while continuing agitation and heating to maintainthe temperature. After aging, acid addition was resumed at the same rateuntil a total of 115.66 grams were added. The total elapsed time foracid addition was 46 minutes. A black slurry was formed with a pH in therange of 3.43 to 3.67. These operations produced 1147.19 grams of blackslurry with a material balance closure at minus 1.95% due toevaporation, handling and transfer losses.

[0089] The black slurry was then filtered and the wet solids washed withhot distilled water for the removal of the soluble sodium sulfate salts.The washed filter cake consisted of 22.45% dry solids and 77.55% water.

[0090] The dry silica/carbon product was comprised of 54.5% amorphousprecipitated silica and 45.5% activated carbon. A solution of 5% solidsin distilled water had a pH of 7.88 and total dissolved salts of 100 ppm(150 uS/cm conductivity). The bulk density was 0.31 g/ml (19.35 poundsper cubic foot). The product had a BET surface area of 267 m²/g, a porevolume of 0.9347 ml/g, and an average pore diameter of 17.0 nanometers.

Example 4

[0091] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon from a dilute RHA sodiumsilicate solution utilizing 37% hydrochloric acid as the acidulatingagent. The RHA sodium silicate solution had the following properties:SiO₂/Na₂O ratio of 3.30 to 1, Na:Si soluble solids at 19.82%(4.61%Na₂O:15.21%SiO₂), diffused carbon (suspended solids) at 12.45%, ina 67.73% water solution, specific gravity of 1.278 and density at 10.66pounds per gallon, with a pH of 11.35. The acidulating agent was 37%hydrochloric acid (HCl) in a water solution.

[0092] The laboratory reactor and equipment used in previous exampleswere utilized in this example.

[0093] The batch production steps were a quantity of 255 ml (200 grams)of RHA sodium silicate was added to the reactor. A quantity of 500 ml(500 grams) of distilled water was added to the silicate solution andthese were agitated while heating to a temperature range of 50°-55° C.The resulting mixture comprised: 5.67%[Na₂O:3.30SiO₂]+3.56% carbonsuspended solids+90.78% water. The pH of the mixture was 10.85. Whilemaintaining the temperature at 50°-55° C. with agitation and heating,37% HCl was added at the rate of 1.85 grams per minute in a 22 minutestime period. Gel formation started after the addition of 40.8 grams ofacid with the reaction mass pH at 8.44. At this point acid addition wasstopped and the black gel mixture was allowed to age for 15 minutes.After aging, acid addition was resumed at the same rate until a total of60.8 grams had been added. The pH of the reaction mass was 2.85. Anadditional quantity of 122.8 grams of the RHA sodium silicate was addedto adjust the pH to 6.06. These operations produced 883.8 grams ofunwashed black slurry with a material balance closure of minus 1.37% dueto evaporation, handling and transfer losses.

[0094] The black slurry was then filtered and the wet solids washed withhot water to remove residual sodium chloride salts produced by the acidreaction with sodium silicate. The washed filter cake consisted of23.56% dry solids and 76.44% water.

[0095] The dry silica/carbon product was comprised of 55.0% amorphousprecipitated silica and 45.0% activated carbon. A solution of 5% solidsin distilled water had a pH of 7.75 and total dissolved salts of 540 ppm(810 uS/cm conductivity). The bulk density was 0.276 g/ml (17.23 poundsper cubic foot). The product had a BET surface area of 264 m²/g, a porevolume of 0.9255 ml/g, and an average pore diameter of 16.8 nanometers.

Example 5

[0096] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon from a dilute RHA sodiumsilicate solution utilizing sulfuric acid as the acidulating agent andslightly different concentration of suspended carbon solids in thestarting silicate solution. The RHA sodium silicate solution had thefollowing properties: SiO₂/Na₂O ratio of 2.93 to 1, Na:Si soluble solidsat 29.75% (7.57%Na₂O:22.18%SiO₂), diffused carbon (suspended solids) at8.15%, in a 62.10% water solution, specific gravity of 1.266 and densityat 10.56 pounds per gallon, with a pH of 11.41.

[0097] The laboratory reactor and equipment used in previous exampleswere utilized in this example.

[0098] The batch production steps were a quantity of 900 ml (1139 grams)of RHA sodium silicate was added to the reactor. No distilled water wasadded to the silicate solution which comprised:29.75%[Na₂O:2.93SiO₂]+8.15% carbon (suspended solids)+62.10% water. ThepH was 11.51. The solution was agitated while heating to a temperaturerange of 50°-55° C. (122°-131° F.). Sulfuric acid was added at a rate of3.0 grams per minute while agitating. Gel formation started afteraddition of 50 grams and a thick gel occurred after addition of 70 gramsof acid. The elapsed time was 23 minutes. The pH of the reaction masswas 8.35. At this point, acid addition was stopped and the black gelmixture was allowed to age for 15 to 20 minutes. After aging, acidaddition was resumed at the same rate until a total of 96 grams had beenadded. The pH of the reaction mass was 4.15. An additional 131 grams ofRHA sodium silicate was added to adjust the pH to 6.95. These operationsproduced 1346 grams of unwashed black slurry with a material balanceclosure of minus 1.46% due to evaporation, handling and transfer losses.

[0099] The black slurry was then filtered and the wet solids washed withhot water to remove the sodium sulfate salts produced by the acidreaction with sodium silicate. The washed filter cake consisted of24.12% dry solids and 75.88% water.

[0100] The dry silica with adhered carbon product was comprised of 73.1%amorphous precipitated silica and 26.9% activated carbon. A solution of5% solids in distilled water had a pH of 6.86 and total dissolved saltsof <10 ppm (<15 uS/cm conductivity). The bulk density was 0.300 g/ml(18.73 pounds per cubic foot). The product had a BET surface area of 199m²/g, a pore volume of 0.6954 ml/g, and an average pore diameter of 9.0nanometers.

Example 6

[0101] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon from a dilute RHA sodiumsilicate solution utilizing 85% o-phosphoric acid as the acidulatingagent. The RHA sodium silicate solution had the following properties:SiO₂/Na₂O ratio of 3.16 to 1, Na:Si soluble solids at 28.38%(6.82%Na₂O:21.56%SiO₂), diffused carbon (suspended solids) at 7.38%, ina 64.24% water solution, specific gravity of 1.265 and density of 10.55pounds per gallon, with a pH of 11.57.

[0102] The laboratory reactor and equipment used in previous exampleswere utilized in this example.

[0103] The batch production steps were a quantity of 200 ml (253 grams)of RHA sodium silicate was added to the reactor. A quantity of 400 ml(400 grams) of distilled water were added to the silicate solution andthese were agitated while heating to a temperature of 50° C. (122° F.).The resulting mixture comprised: 10.99%[Na₂O:3.16SiO₂]+2.86% carbonsuspended solids+86.15% water. The pH of the mixture was 11.30. Whilemaintaining the temperature at 50° C. with agitation and heating, 85%H₃PO₄ was added at the rate of 1.10 grams per minute in a 15 minutestime period. Gel formation stated after addition of 16 grams of acidwith reaction mass pH at 8.97. At this point acid addition was stoppedand the black gel was stirred and aged for 10 minutes. After aging, acidaddition was resumed at the same rate until a total of about 28 gramshad been added. The pH of the reaction mass was 7.03. The mass was agedfor 30 minutes while stirring before filtration was initiated. Theseoperations produced 665 grams of unwashed black slurry with a materialbalance closure of minus 2.35% due to evaporation, handling and transferlosses.

[0104] The black slurry was then filtered and the wet solids washed withhot water to remove the sodium phosphate salts produced by the acidreaction with sodium silicate. The washed filter cake consisted of22.56% dry solids and 77.44% water.

[0105] The dry silica/carbon product was comprised of 74.5% amorphousprecipitated silica and 25.5% activated carbon. A solution of 5% solidsin distilled water had a pH of 7.34 and total dissolved salts of 480 ppm(720 uS/cm conductivity). The bulk density was 0.461 g/ml (28.77 poundsper cubic foot). The product had a BET surface area of 195 m²/g, a porevolume of 0.6833 ml/g, and an average pore diameter of 12.4 nanometers.

Example 7

[0106] This example illustrates the production of precipitated amorphoussilica with adhered or deposited carbon from a dilute RHA sodiumsilicate solution utilizing a larger bench sized reactor of about 19liters volume (5 gallons) and a membrane filter unit. The purpose was toobtain information on scale-up factors for commercial equipment designand selection. The reactor was equipped with a variable speed mixer andelectric heating bands for temperature control. The RHA sodium silicatesolution had the following properties: SiO₂/Na₂O ratio of 2.12 to 1,Na:Si soluble solids at 24.09% (5.85%Na₂O:18.24%SiO₂), diffused carbonsuspended solids at 6.00%, in a 69.91% water solution, specific gravityof 1.213 and density of 10.12 pounds per gallon, with a pH of 11.28. Theacidifying agent was 85% o-phosphoric acid.

[0107] The batch production steps were the reactor was initially loadedwith 16.74 kg (36.91 lbs.) of RHA sodium silicate solution whileagitating and heating to 55° C. (131° F.). After reaching the reactiontemperature, the phosphoric acid was added at the rate of 0.14 kg perminute (0.31 lbs/min) in a 15 minutes time period. Gel formation wasobserved at this time and the acid addition was stopped. The pH of thereaction mass was 7.13. The black gel was aged for 30 minutes withagitation at 55° C. These operations produced 18.8 kg (8.53 lbs) ofunwashed black slurry. The black slurry was then transferred to themembrane filter unit and washed with hot water until the sodiumphosphate salts were essentially removed. The pH was 7.65 and the totaldissolved salts were <200 ppm. The washed filter cake consisted of23.55% dry solids and 76.45% water. The washed solids were then vacuumedfiltered and dried in a convection oven at 110° C. for 3 to 4 hours.

[0108] The dry silica/carbon product was comprised of 75.28% amorphousprecipitated silica and 24.72% activated carbon. A solution of 5% solidsin distilled water had a pH of 7.55 and total dissolved salts of 160 ppm(230 uS/cm conductivity). The bulk density was 0.441 g/ml (27.56 poundsper cubic foot). The product had a BET surface area of 193 m²/g, a porevolume of 0.6762 ml/g, and an average pore diameter of 12.3 nanometers.TABLE 1 PRECIPITATED SILICA WITH ADHERED CARBON PROPERTIES PropertyExample 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7Silica wt % 66.14 93.63 54.50 54.99 73.13 74.49 75.28 Carbon wt % 33.866.37 45.50 45.01 26.87 25.51 24.72 Silica/Carbon Ratio 1.95/1 14.7/11.20/1 1.22/1 2.72/1 2.92/1 3.05/1 Wet Cake Moisture, WCM % 79.27 78.4477.55 76.44 75.88 77.44 76.45 Filter Cake Solids, FCS % 20.73 21.5622.45 23.56 24.12 22.56 23.55 Structure Index (WCM/FCS × 100) 382 364345 324 315 343 325 pH 6.95 6.38 7.88 7.75 6.95 7.34 7.55 Totaldissolved salts, ppm <10 <10 100 540 <10 480 160 Conductivity, uS/cm <15<15 150 810 <15 720 230 Bulk density, g/ml 0.30 0.37 0.31 0.28 0.30 0.460.44 PCF pounds/cubic foot 18.73 23.10 19.35 17.48 18.73 28.71 27.46 BETsurface area, m²/g 220 155 267 264 199 195 193 Pore volume, ml/g 0.77070.5429 0.9347 0.9255 0.6964 0.6833 0.6762 Average pore diameter, nm 14.09.9 17.0 16.8 9.0 12.4 12.3 DBP absorption, ml/100 g 183 129 223 220 166163 161

[0109] Examples 8-10 are directed to the production of precipitatedamorphous silica without adhered or deposited carbon solids from dilutesodium silicate solution derived from caustic digestion of rice hull ashafter the diffused activated carbon particles have been removed byfiltration. The results are provided in Table 2 following Example 10.

Example 8

[0110] This example illustrates the production of pure precipitatedamorphous silica without adhered or deposited amorphous carbon. A RHAsodium silicate liquid sample was filtered to remove all the carbonsuspended solids prior to acidification. The clear, dilute sodiumsilicate solution had the following properties: SiO₂/Na₂O ratio at 3.02to 1, Na:Si total soluble solids at 28.71% (7.14%Na₂O:21.57%SiO₂), in a71.29% water solution, specific gravity of 1.245 and density at 10.38pounds per gallon, with a pH of 11.46. The acidulating agent utilizedwas concentrated sulfuric acid, 96.5% in aqueous solution.

[0111] The laboratory reactor and equipment used in Example 1 wereutilized to produce the pure precipitated silica product.

[0112] The batch production steps included a quantity of 100 ml (124.5grams) of clear dilute RHA sodium silicate solution were added to thereactor. A quantity of 573 ml (573 grams) of distilled water were addedto the silicate solution and the agitator was started while heating to50°-55° C. (122°-131° F.). The resulting mixture comprised:5.12%[Na₂O:3.02SiO₂]+94.88% water. While maintaining agitation andtemperature in the above range, sulfuric acid was added at the rate of2.68 grams per minute over a 37 minutes period. The acid addition wasstopped and the white gel was allowed to age for 15 minutes. The slurrypH was 2.34. Additional clear sodium silicate was added at the rate of13.25 grams per minute for a 45 minutes period to adjust the pH to 7.49.The white slurry was then allowed to age for a period of 20 minuteswhile agitating. The above operations produced 1399 grams of whiteslurry with a pH of 7.58.

[0113] The white slurry was then filtered and washed with hot water toremove the sodium sulfate salts. The washed filter cake consisted or22.42% dry solids and 77.58% water.

[0114] The wet precipitated silica product was dried in a convectionoven for about 5 hours at 110° C. A solution of 5% solids in distilledwater had a pH of 7.84 and total dissolved salts of 60 ppm (90 uS/cmconductivity). The bulk density was 0.337 g/ml (21.04 pounds per cubicfoot). The product had a BET surface area of 205 m²/g, a pore volume of0.7181 ml/g, and an average pore diameter of 13.0 nanometers.

Example 9

[0115] This example illustrates the production of pure precipitatedamorphous silica without adhered or deposited amorphous carbon utilizingphosphoric acid for acidification of a clear, dilute sodium silicateafter filtration removal of the suspended carbon solids. The clear,dilute sodium silicate solution had the following properties: SiO₂/Na₂Oratio of 3.85 to 1, Na:Si total soluble solids at 18.97%(3.91%Na₂O:15.06%SiO₂), in a 81.03% water solution, specific gravity of1.128 and density at 9.41 pounds per gallon, with a pH of 11.27.

[0116] The laboratory reactor and equipment were utilized in thisexample.

[0117] The batch production steps were a quantity of 230 ml (259 grams)of clear dilute RHA sodium silicate were added to the reactor withagitation and heating to 50° C. Phosphoric acid was added at 2.25 gramsper minute over a 15 minutes period. Gel formation was observed at theend of the period and the pH was 6.89. The white gel was allowed to agefor a 20 minutes period while agitating and heating. The aboveoperations produced 293 grams of white slurry with a pH of 6.95.

[0118] The white slurry was then filtered, washed with hot water toremove sodium phosphate salts, and dried in a convection oven. Thewashed filter cake consisted of 23.62% dry solids and 76.38% water.

[0119] A 5% solution of dry precipitated silica in distilled water had apH of 7.23 and total dissolved salts of 540 ppm (810 uS/cmconductivity). The bulk density was 0.472 g/ml (29.46 pounds per cubicfoot). The precipitated silica product had a BET surface area of 287m²/g, a pore volume of 0.9943 ml/g, and an average pore diameter of 18.0nanometers.

Example 10

[0120] This example illustrates the production of pure precipitatedamorphous silica without adhered or deposited amorphous carbon utilizinghydrochloric acid for acidification of a clear, dilute sodium silicateafter filtration removal of the suspended carbon solids. The clear,dilute sodium silicate solution had the following properties: SiO₂/Na₂Oratio of 2.77 to 1, Na:Si total soluble solids at 29.05%(7.71%Na₂O:21.34%SiO₂), in a 70.95% water solution, specific gravity of1.205 and density at 10.05 pounds per gallon, with a pH of 11.38.

[0121] The laboratory reactor and equipment were utilized in thisexample.

[0122] The batch production steps were a quantity of 200 grams of cleardilute RHA sodium silicate were added to the reactor with agitation andheating to 50° C. Hydrochloric acid was added at 1.0 grams per minutefor 27 minutes, at which time the first gel formation was observed. Thereaction mass pH was 9.04. The mass was allowed to age under agitationfor 15 minutes. An additional 52 grams of dilute sodium silicate wereadded while agitating and maintaining temperature at 50° C. Anadditional 11 grams of hydrochloric acid were added to adjust pH to5.91. The white slurry was allowed to age for 60 minutes prior tofiltration. The above operations produced 290 grams of white slurry witha pH of 6.17.

[0123] The white slurry was filtered, washed with hot water to removethe sodium chloride salts, and dried in a convection oven. The washedfilter cake consisted of 24.12% dry solids and 75.88% water.

[0124] A 5% solution or dry precipitated silica in distilled water had apH of 7.09 and total dissolved salts of 380 ppm (570 uS/cmconductivity). The bulk density was 0.4194 g/ml (26.18 pounds per cubicfoot). The precipitated silica product had a BET surface area of 258m²/g, a pore volume of 0.8990 ml/g, and an average pore diameter of 16.0nanometers. TABLE 2 PRECIPITATED SILICA WITHOUT ADHERED CARBONPROPERTIES Property Example 8 Example 9 Example 10 Silica wt % 100 100100 Wet Cake Moisture, WCM % 77.58 76.38 75.88 Filter Cake Solids, FCS %22.42 23.62 24.12 Structure Index (WCM/FCS × 100) 346 323 315 pH 7.847.23 7.09 Total dissolved salts, ppm 60 540 380 Conductivity, uS/cm 90810 570 Bulk density, g/ml 0.337 0.472 0.419 PCF pounds/cubic foot 21.0429.46 26.15 BET surface area, m²/g 205 287 258 Pore volume, ml/g 0.71810.9943 0.8990 Average pore diameter, nm 13.0 18.0 16.0 DBP absorption,ml/100 g 171 239 215

[0125] Precipitated silica solutions for potential use in CMP polishingslurries were formulated by mixing the pure precipitated silicaparticles obtained by acidifying sodium silicate solutions derived fromcaustic digestion of biomass ash (rice hull ash), with appropriatequantities of deionized water. Metal analyses were conducted by IonCoupled Argon Plasma (ICP) and the results are compared to twocommercially available CMP slurries, designated A and B in Table 3. Itis noted that the commercial CMP slurries contain silica precipitatedfrom commercial sodium silicates and were further processed through ionexchange resins to remove metal contaminants, while the Examples 11, 12and 13 samples were precipitated from sodium, silicate solutions derivedfrom the caustic digestion of rice hull ash and were not treated by anymetals removal technique. In other words, the Examples 11, 12 and 13silica slurries contain the metals that were present in the startingrice hull ash raw materials, since no metals removal treatments wereemployed. The ICP analytical results were adjusted to equalize thesilica composition of the slurries and are reported inparts-per-million, ppm. TABLE 3 Metals Analyses Results - CMP slurriesMetals Example 11 Example 12 Example 13 Commercial A Commercial B Na19.8 18.6 18.7 19.4 20.1 Ca 22.9 23.1 19.9 12.8 13.2 Mg 3.8 4.9 2.2 9.49.7 Al 34.0 28.0 26.0 87.0 90.0 Cr 0.1 0.2 0.2 0.6 0.5 Cu 0.1 0.1 0.14.3 4.1 Fe 3.3 2.9 2.2 23.4 16.2 K 4.7 3.6 3.5 4.7 4.1 Sn 0.3 0.7 0.51.8 1.5 Sr 0.3 0.2 0.1 0.6 0.5 Ti 1.3 1.5 1.1 72.8 75.3 Zn 6.3 4.5 4.81.6 1.3 Zr 0.5 0.4 0.1 17.2 14.3 Totals 97.4 88.7 79.4 255.6 250.8

[0126] These results clearly show that the precipitated silicas producedfrom sodium silicate solutions derived from caustic digestion of biomassash (rice hull ash) have metals impurities that are less than half theconcentration levels in silicas produced from commercial sodium silicatesolutions. The biomass ash derived sodium silicates producedprecipitated silicas have 260% to 319% lower metals content, without anyfurther removal treatments being employed.

[0127] Other agricultural byproducts or waste materials having varyingquantities of biogenic silica, that is, silica which is developed,assimilated or occurs in the cell structures of living organisms such asplants are useful in the present invention. As previously mentioned,these products are commonly referred to as biomass and are principallyrice hulls, rice straw;, wheat straw, sugarcane, bagasse, esquisitum(horsetail weeds), certain palm leaves (palmyra palm), and bamboo stemsprocessed the same as in Examples 1-10 produce satisfactory amorphoussilicas, silica gels, with (Examples 1-7) or without (Examples 1-10)adhered or deposited amorphous carbons for the particular uses of themas previously set forth herein.

[0128] Accordingly, the present invention is well suited and adapted toattain the ends and carry out the objects set forth and has theadvantages and features mentioned as well as others inherent therein

[0129] While presently preferred examples of the embodiments of theinvention have been given for the purposes of disclosure, changes can bemade therein which are within the spirit of the invention as defined bythe scope of appended claims.

What is claimed is:
 1. A method of producing amorphous precipitatedsilicas, silica gels with adhered or deposited amorphous carbonscomprising, acidifying a caustic silicate solution produced by causticdigestion of biomass ash containing silica and activated carbon, the ashbeing obtained from thermal pyrolysis of the biomass, the acidifyingeffective to produce a slurry of the precipitated silicas and silicagels with the adhered or deposited amorphous carbons, and separatingfrom the slurry the precipitated silicas and silica gels with theadhered or deposited amorphous carbons.
 2. The method of claim 1 where,the biomass ash comprises rice hull ash.
 3. The method of claim 1 where,the acidifying is by an acid selected from the group consisting ofsulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, aceticacid, and combinations thereof.
 4. A method of producing amorphousprecipitated silicas, silica gels comprising, acidifying a causticsilicate solution produced by caustic digestion of biomass ashcontaining silica and free of activated carbon, the ash being obtainedfrom thermal pyrolysis of the biomass, the acidifying effective toproduce a slurry of the precipitated silica and silica gels, andseparating from the slurry the precipitated silicas and silica gels. 5.The method of claim 4 where, the biomass ash is rice hull ash.
 6. Themethod of claim 4 where, the acidifying is by an acid selected from thegroup consisting of sulfuric acid, hydrochloric acid, phosphoric acid,nitric acid, acetic acid, and combinations thereof.
 7. Amorphousprecipitated silica or silica gel with adhered or deposited amorphouscarbon in natural state from a slurry of them produced by acidifying acaustic silicate solution produced by caustic digestion of biomass ashcontaining silica and activated carbon.
 8. The amorphous precipitatedsilica or silica gel of claim 7 where, the biomass ash comprises ricehull ash.
 9. Amorphous precipitated silica or silica gel free ofamorphous carbon separated from a slurry of them produced by acidifyinga caustic silicate solution produced by caustic digestion of biomass ashcontaining silica and from which activated carbon had been removed, theprecipitated silica and silica gel having total metal contaminants notexceeding about 250 ppm.
 10. The amorphous precipitated silica or silicagel of claim 9 where, the biomass ash comprises rice hull ash.